I believe this change makes it much easier for visitors and casual readers to read the question and get right to the answers. Questions attract a lot of pseudo-answers and chit-chat in comments, and even the comments that are "what comments are for" are generally only helpful for the OP (because they're requests for clarification). For other comments that should be seen, like links to related questions, voting on the comment mitigates.
For regulars and readers who want to see everything, the full comments are one click away and don't even require a page reload (it's an inline change). That's low-cost. People who are about to add more comments to a collapsed, busy thread -- who must have at least 50 rep to do so, so we're not talking new accounts from Google here -- are automatically presented with the full batch of comments first. They might not read them, but they aren't going to add comment #17 without noticing. I haven't seen signs that people are leaving more poor comments than they used to because of the initial collapse.
I would slightly prefer it if this collapsing applied only to comments on the question but not on the answers. Comments that point out problems in answers are not mainly for the author, unlike requests for clarification on the question, so they should be seen without having to click for each answer. However, a single comment vote makes these visible, so this is not a big deal. Were you to raise the number of votes needed, though, that could be a problem.
I haven't seen a lot of "tactical voting", e.g. people voting up one side of an argument to get it above the fold. That's good.